It, #, is also used in musical notation to idicate that a note should be sharp. The musical scale includes 12 notes. Some combinations of notes are not in harmony with each other, so typically you use only 7 of the twelve in a given piece of music. To keep things simple, they only named 7 notes (ABCDEFG), and placed the other five inbetween the seven. The # sign is used to show that the note above the current note is to be used, so an F# would actually use one of the 'extra' five notes that lies between F and G. A flat is the opposite (ie, a G flat is the same as an F sharp)

Also pronounced hash (my personal pronunciation), octothorpe if you want to confuse people, or sha, as in shabang (#!), and AFAIK only when used there. Brits don't pronounce it 'pound', because then we'd mix it up with the UK pound symbol, '£'.

The # sign (approximated), also known as the sharp can appear amid a song (an accidental), or in the key signature. Any note that falls on the note modified by the sharp will cause any previous accidental or key signature (for that note) to be considered moot and the new, sharped, value to take its place. The value is then raised a half-stepharmonically.

A # drawn anywhere can also be used as the playing field for the popular ultra-portable game of tic-tac-toe, in which two players take turns battling it out with x's and o's to see who can get three in a row.

See tic-tac-toe for a painfully in-depth description of the game, including strategies.